When a voltage is applied to an organic electroluminescence device (also hereinafter referred to as an organic EL device), holes and electrons are injected into an emitting layer from an anode and a cathode, respectively. In the emitting layer, the injected holes and electrons are recombined to form excitons. At this time, according to electron spins statistics, singlet excitons and triplet excitons are generated in a rate of 25%:75%. The emitting type is classified into two groups, i.e. fluorescent type and phosphorescent type, according to the emitting system. In the case of fluorescent type, due to using the emitting by singlet excitons, the internal quantum efficiency is regarded as having a limitation of 25%. On the other hand, in the case of phosphorescent type, emission by triplet excitons is used. As a result, it is known that if the intersystem crossing from singlet excitons occurs efficiently, the internal quantum efficiency can be increased to 100%.
Traditionally, for an organic EL device, suitable device designing has been made depending on the emitting mechanism, i.e. fluorescent type emission or phosphorescence type emission. In particular, as for a phosphorescent organic EL device, it is known that a high-performance device cannot be obtained by simple application of the technology of a fluorescent device due to its emission properties. The reason therefor is generally assumed to be as follows.
Since phosphorescent emission is emission utilizing triplet excitons, the energy gap of a compound used in the emitting layer must be large. The reason therefor is that, the singlet energy (i.e. a difference in energy between the lowest excited singlet state and the ground state) of a certain compound is normally larger than the triplet energy (i.e. a difference in energy between the lowest excited triplet energy state and the ground state) of the compound.
Therefore, in order to efficiently confine the triplet energy of the phosphorescent dopant material in the device, it is required to use a host material having a triplet energy larger than the triplet energy of the phosphorescent dopant material. Further, when an electron-transporting layer and a hole-transporting layer are provided in adjacent to the emitting layer, a compound having a triplet energy larger than that of the phosphorescent dopant material must be used in the electron-transporting layer and the hole-transporting layer. In this way, based on the conventional device design concept, as compared with a compound used in a fluorescent organic EL device, a compound having a further larger energy gap is used in a phosphorescent organic EL device, whereby a driving voltage in the entire organic EL device is increased.
Moreover, hydrocarbon-based compounds having high oxidative resistance and reductive resistance, which are useful for fluorescent devices, have a wide broadening of pi-cloud, and thus have a small energy gap. Hence, for a phosphorescent organic EL device, such hydrocarbon-based compounds can hardly be selected, and organic compounds containing hetero atoms such as oxygen or nitrogen tend to be selected. As a result, a phosphorescent organic EL device has a disadvantage of a shorter life time as compared with a fluorescent organic EL device.
In addition, the relaxation rate of triplet excitons of phosphorescent dopant materials is much longer than that of singlet excitons. This influences the device performance. That is, the emission from singlet excitons has a rapid relaxation rate resulting in emission, and thus diffusion of excitons into neighboring layers of the emitting layer (hole-transporting layer and electron-transporting layer, for example) hardly occurs, whereby efficient emitting can be expected. On the other hand, emission from triplet excitons is slowly relaxed due to the spin-forbidden principle, and thus diffusion of excitons into neighboring layers occurs easily. As a result, thermal energy deactivation occurs from other than the specific phosphorescent emitting compounds. That is, as compared with fluorescent organic EL device, it is more important to control the region where electrons and holes are recombined.
For the reasons mentioned above, in order to obtain a high-performance phosphorescent organic EL device, it is required to select different materials from those used in a fluorescent organic EL device and to design differently from the fluorescent organic EL device.
As such an organic EL material, traditionally, carbazole derivatives, which are known for their high triplet energy and as a main skeleton of a hole-transporting material, have been used as a useful phosphorescent host material.
Patent Document 1 discloses that a compound containing a carbazole skeleton and a nitrogen-containing heterocyclic group in the same molecule is used as an organic EL device material. The compound is molecularly designed such that carrier transporting is balanced, by introducing an electron-deficient nitrogen-containing heterocyclic group to the hole-transporting carbazole skeleton.
However, the life of an organic EL device is required to be further improved, and it is desired to develop an organic EL device material capable of realizing a more long life time.